Magnetic tape recording is used not only for analog but also for digital signals. Digital signal recording on magnetic tape is used widely in connection with data processing apparatus. Increasingly, the recording of video signals is carried out by converting received analog video signals into digital form, and recording the digitized video signals on magnetic tape on inclined tracks.
Various types of codes can be used for recording digital signals. Basically, two types of codes are used, one which is free from a d-c level, and another one which includes a d-c level. In the d-c level codes, for example the NRZ code, it is possible that sequences of bits of the same polarity may cause a shift in the d-c level upon processing of the data. For example, if twenty or more bits of the same polarity, that is, logic 0 or logic 1 appear sequentially, shift of the base line, that is, of the reference voltage level, may occur. Such drift phenomena then can cause difficulty if a subsequent bit of a different polarity occurs, since the base line has been shifted by the extensive sequence of similar bits to such an extent that the control of the magnetic recording medium, typically magnetic tape, is distorted in such a manner that the desired positive or negative magnetization to saturation of the magnetic medium can no longer be reached. D-C codes have the advantage, however, that processing of the data is simplified and requires less equipment and apparatus than processing of similar data in codes without a d-c content.